Doping-induced Polaron Formation and Solid-state Polymerization in

Article Cite This: J. Phys. Chem. C 2017, 121, 24397-24407

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Doping-Induced Polaron Formation and Solid-State Polymerization in Benzoporphyrin−Oligothiophene Conjugated Systems Dmytro Solonenko,† Jacek Gasiorowski,† Dogukan Apaydin,‡ Kerstin Oppelt,§ Martin Nuss,∥ Wittawat Keawsongsaeng,⊥ Georgeta Salvan,† Kurt Hingerl,# Niyazi Serdar Sariciftci,‡ Dietrich R. T. Zahn,† and Patchanita Thamyongkit*,⊥,∇ †

Semiconductor Physics, Chemnitz University of Technology, Reichenhainer Strasse 70, 09107 Chemnitz, Germany Linz Institute for Organic Solar Cells (LIOS), Institute of Physical Chemistry, §Institute of Inorganic Chemistry, and #Center for Surface and Nanoanalytics, Johannes Kepler University Linz, Altenbergerstrasse 69, 4040 Linz, Austria ∥ Institute of Theoretical and Computational Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria ⊥ Department of Chemistry, Faculty of Science and ∇Research Group on Materials for Clean Energy Production STAR, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

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‡

S Supporting Information *

ABSTRACT: Benzoporphyrins and their derivatives are of high interest in organic semiconductor technology due to their peculiar physical properties valuable for optoelectronic applications. Following our previous work successfully developing meso-thienyl- or meso-bithiophenyl-substituted zinc benzoporphyrins as efficient ternary components for bulk heterojunction solar cells, we describe herein detailed spectroscopic studies on doping of solid films of these benzoporphyrins under iodine atmosphere. Solid-state doping and oxidative polymerization are investigated by Raman and Fourier transform infrared spectroscopy. Structural and vibrational changes upon doping are explored with supporting data from density functional theory calculations. Furthermore, the optical and spectroscopic characteristics of the films of these materials are also monitored during the doping, and the polaron formation as evidenced by in situ attenuated total reflection Fourier transform infrared and UV−vis spectroscopy is observed. These results suggest that the target zinc benzoporphyrins, both in monomeric and in polymeric forms, should be good candidates in several other optoelectronic applications.

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INTRODUCTION Among many organic materials for optoelectronic and (photo)electrocatalytic applications, porphyrins and phthalocyanines area focus of current research due to their desirable specific chemical, optical, and electrical properties as well as their ubiquity in nature.1−3 Their applicability in optoelectronic and bio- and electrochemical devices, such as organic solar cells,4−13 dye-sensitized solar cells,14−22 field effect transistors, organic light-emitting diodes,23−30 optical,31−38 chemo-, and biosensors,39−45 was already extensively proven. However, like the majority of organic materials, the most critical limitation for the practical usage of porphyrins and phthalocyanines in devices resides in their intrinsic limited charge carrier mobility within thin films of these molecules (approximately 10−10−101 cm2·(V−1 s−1)46), compared with inorganic semiconductors (approximately 102−104 cm2·(V−1 s−1)46). Additionally, the high aggregation tendency of these materials adversely affects the morphology of films blended with other organic or inorganic molecules, resulting in a reduced performance of the optoelectronic applications. In order to overcome such disadvantages of organic molecules, one of the most popular approaches to develop highly efficient devices is the use of © 2017 American Chemical Society

conjugated conducting polymers. It would be therefore desirable to find a route toward polymerization of the porphyrins and phthalocyanines which would allow their advantageous optical and catalytic properties to be combined with the high charge carrier mobility of polymers. For this purpose, functionalization of these kinds of compounds with suitable functional groups has to be addressed. Owing to their high absorption coefficients and relatively high conductivities of up to 10 S·cm−1,47 polythiophene derivatives have already proved their applicability in organic solar cells, organic field effect transistors, organic light-emitting diodes, and photo(electro)chemical catalysis.48−50 In addition, detailed studies were also carried out on their optical and electrical properties as well as on their photo(electro)chemistry.51 Due to the induced delocalization of generated free charge carriers along the conjugation length, the high conductivity in such systems usually results from polaron formation,52 which drastically changes the electronic structure Received: July 10, 2017 Revised: September 18, 2017 Published: October 16, 2017 24397

DOI: 10.1021/acs.jpcc.7b06771 J. Phys. Chem. C 2017, 121, 24397−24407

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The Journal of Physical Chemistry C Chart 1. Molecular Structures of the Benzoporphyrins Investigated

applications of benzoporphyrin materials in the fields of optoelectronics and heterogeneous photo(electro)catalysis.

of the polymers and, thus, their optical and electrical properties. The spectroscopic appearance of polarons is measured as a change in the optical spectrum, i.e., a decrease of the initial absorption and appearance of two new transitions with energies lower than the optical band gap.53 Therefore, functionalization of porphyrins/phthalocyanines with thiophene-containing side groups might provide a suitable approach to porphyrin polymerization. In this work, solid-state iodine (I2) treatment of Zn− benzoporphyrins bearing thienyl (Zn−TTBP) or bithiophenyl meso substituents (Zn−T2TBP), structures of which are presented in Chart 1, was investigated. Benzoporphyrins are the porphyrinic analogues of phthalocyanines that can be synthesized by several well-developed synthesis routes for various structural modifications. Recently, Zn−TTBP and Zn− T2TBP were reported to remarkably improve the performance of inverted ternary bulk heterojunction solar cells when used as additives in photoactive organic layers,54 and therefore, it is interesting to further investigate their detailed spectroscopic and electronic behavior, especially upon doping, to gain behavioral and mechanistic insight into their photovoltaic performance. I2 treatment is known as a popular oxidation/doping method for several kinds of polymer films as a result of the active charge and/or radical formation in the films55 and, thus, can be used in the polymerization process. In general, the doped materials can subsequently be either dedoped, leading to the recovery of the original forms, or polymerized. In the latter case, the presence of I2 results in chemical oxidation of the material with incorporation of I3− counterions.55 Therefore, to facilitate the solid-state polymerization, such monomer solid films should have particular conditions, such as (i) an appropriate orientation of the molecules, (ii) sufficiently long lifetime, which is corroborated with (iii) a high concentration of free charges and/or radicals, etc. In this study, changes in vibrational, molecular, and electronic structures of the Zn− benzoporphyrin derivatives were complementarily monitored using Raman and infrared (IR) spectroscopic techniques. The resulting spectra were compared with those obtained from density functional theory (DFT) calculations and with those of meso-unsubstituted Zn−benzoporphyrin (Zn−BP) used as a reference material. The formation of the polarons was investigated using in situ IR and ultraviolet (UV)−vis techniques and confirmed by ex situ spectroscopic ellipsometry. The conclusions drawn from these studies should help to develop an approach for the preparation of stable benzoporphyrin-based polymer films. This will pave the way to

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EXPERIMENTAL METHODS Materials and Methods. All chemicals were analytical grade, purchased from commercial sources, and used as received unless noted otherwise. Noncommercial Compounds. All benzoporphyrins, i.e., Zn−BP, Zn−TTBP, and Zn−T2TBP, were obtained from synthetic procedures described in our previous report, and therefore, their 1H and 13C NMR spectra can be found therein as well.54 Films of Zn−BP, Zn−TTBP, and Zn−T2TBP were prepared by drop casting a solution of the pristine molecules in tetrahydrofuran (THF, 5 g·L−1) on clean 15 mm × 15 mm ITO-coated glass substrates. Substrates were cleaned by consecutive ultrasonic bathing in acetone (10 min), isopropanol (20 min), and deionized water (15 min) and followed by immediate drying with a nitrogen flow. Raman measurements of the resulting thin films were performed at room temperature using a Horiba LabRAM HR800 micro-Raman spectrometer with a 514.7 nm line of a DPSS Cobolt laser. Spectral resolution is below 3 cm−1. An area with a diameter of approximately 1 μm was probed by a 50xLWD (long working distance) objective lens. The incident power on the sample surface was adjusted in the range below 100 μW to avoid possible photoeffect(s), resulting in a temperature increase and degradation of the materials, but still to be sufficient to obtain reasonable peak intensities. The Raman measurements of powders were recorded by focusing on flat planes of microcrystallites. The measurements, performed before and after I2 treatment, were carried out at room temperature by exposing the benzoporphyrin films to I2 vapor from a small I2 crystal (approximately 1 mg) that was placed nearby (approximately 1 cm away) in the same closed glass Petri dish having dimensions of 60 mm × 17 mm (diameter × height). Removal of excessive I2 from the films was achieved ex situ by heating the films in an open chamber on a hot plate at a constant temperature of 200 °C for 15 min. Completion of this treatment was visually indicated by a color change of the films compared with their original colors. IR spectra were recorded at room temperature using a Fourier transform infrared (FTIR) spectrometer (Bruker IFS66S) in an attenuated total reflection (ATR) mode. The measurements were performed using a mercury−cadmium telluride (MCT) detector cooled with liquid nitrogen prior to the measurements. For all ATR-FTIR measurements, a 24398


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The Journal of Physical Chemistry C precleaned 45° ZnSe crystal having dimensions of 10 mm × 10 mm × 1 mm (width × length × thickness) was used as a reflection element. A precleaning process was achieved by consecutive polishing the ZnSe crystal with 1 and 0.25 μm diamond paste and then washing with refluxing acetone for 3 h. The 5 g·L−1 solutions of the benzoporphyrin derivatives in THF were drop casted on the ZnSe crystals with the film thickness ranging between 100 and 120 nm. After that the resulting films were assembled into a closed Teflon compartment having dimensions of 1.0 cm × 1.0 cm × 5.5 cm (width × length × height). I2 treatment was performed by dropping a small I2 crystal (approximately 1 mg) 0.2−0.3 mm in front of the benzoporphyrin-coated ZnSe crystal in the Teflon compartment through a small opening (2.5 mm diameter) which was immediately closed afterward. The spectra were recorded every 20 s. The ATR-FTIR spectra of the untreated (undoped) films were used as reference spectra (Tref) to compare with those of the treated films (Ts) in order to obtain spectral changes during the treatment. UV−vis spectra were recorded with a Varian Cary 50 spectrophotometer. The measurements were performed on the films that were prepared by drop casting the 5 g·L−1 solutions of the benzoporphyrin derivatives in THF onto an inner wall of a cuvette. After collecting an absorption spectrum of the pristine film, the I2 crystal was dropped into the cuvette which was immediately tightly closed. The absorption spectra were instantly collected every 30 s, allowing one to detect the changes upon chemical oxidation of the film in the presence of the I2 vapor subliming from its crystal at room temperature. Near IR (NIR)−vis−UV ellipsometric characterization was performed using a Woollam M-2000 rotating compensator ellipsometer, which spanned an energy range from 0.73 to 6.5 eV. Dielectric functions were obtained by fitting measured ellipsometric response, ψ and Δ, using the complex dispersion relation in a WVASE software.56 The DFT calculations were performed using Gaussian 09 software.57 For each molecule, geometry optimization and frequency determination were carried out using a B3LYP functional58 in conjunction with a LANL2DZ basis set.59−62 Three-dimensional (3D) molecular structures were prepared with CYLview.63 The highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are visualized with an isovalue of 0.02 and density of 0.0004.

Figure 1. Calculated top-view (left images) and side-view (middle images) molecular structures and electron density of HOMO and LUMO states (right images) of the (a) Zn−BP, (b) Zn−TTBP, and (c) Zn−T2TBP molecules.

geometry and, thus, the vibrational structure. Unlike the Zn− BP molecule which is planar (Figure 1a), the DFT calculations show that macrocyclic cores of the benzoporphyrin derivatives, Zn−TTBP and Zn−T2TBP, are strongly bent away from the planar configuration due to interaction with the meso-aryl substituents. If a line from one meso-aryl group via Zn to the opposite meso-aryl group defines an x axis and a perpendicular one is y then curvature in a +z direction along the x axis implies a curvature in the −z direction along the y axis. Furthermore, the meso-aryl groups are tilted with respect to the Zn−BP macrocycle. Bending of a central element distinguishes the meso-substituted benzoporphyrins strongly from the porphyrin and phthalocyanine analogues already on the level of the molecular structure. Moreover, the meso substitution and the molecular curvature cause a significant change in the length of several bonds throughout the molecules, for example, the Zn−N bond length which is decreased from 2.09 (Zn−BP) to 2.07 Å (Zn−TTBP and Zn−T2TBP), Cα−Cm bond length which is increased from 1.37 (Zn−BP) to 1.42 Å (Zn−TTBP and Zn−T2TBP), and so on (see Supporting Information, Chart S1/Table S1). The induced curvature of the molecules additionally leads to structural anisotropy as the bending happens in opposite directions within a z axis. This has an impact on the vibrational as well as the electronic structures of the meso-substituted benzoporphyrins with respect to those of the planar Zn−BP molecule. When looking at the simulated distributions of molecular orbitals, the results indicate that neither the HOMO nor the LUMO of the calculated benzoporphyrins exhibits charge density at the central Zn atoms. All considered benzoporphyrin derivatives have qualitatively similar HOMOs and LUMOs with only slight differences due to negligible interaction with their meso-aryl groups. The results show that their HOMOs are located mostly on all meso and pyrrolic carbons but not on N atoms. The LUMOs, however, are mainly

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RESULTS AND DISCUSSION 1. Molecular Structure and Electronic Orbitals. First, the DFT calculations were performed by starting with geometry optimization of the molecular structures of the benzoporphyrins. In a second step, electronic configurations and vibrational patterns of the molecules were determined. A B3LYP hybrid functional was chosen since it showed excellent performance for proportional consideration of exact and empirical contributions describing exchange and correlation of electrons in the system.58 The LanL2DZ basis set was chosen for a good description of a 3d central metal atom (in this case, Zn) using the effective core potential approximation.59−62 The optimized 3D molecular structures of Zn−BP, Zn−TTBP, and Zn− T2TBP are shown in Figure 1. The calculated molecular structures are in a good agreement with the results obtained using a 6-31g(d,p) basis set (see Supporting Information, Chart S1/Table S1). The good agreement for both types of calculations suggests the appropriateness of the optimized 24399


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Figure 2. (a) Raman spectra of the Zn−BP, Zn−TTBP, and Zn−T2TBP powders. All spectra are normalized with respect to the intensity of the most intense mode and corrected with respect to the baseline. Raman and IR spectra of the (b) Zn−TTBP and (c) Zn−T2TBP thin films. Dashed lines highlight the spectral modes, which are both IR and Raman active.

the vibrational modes in the Raman and IR spectra was performed (for full vibrational assignment, see Tables S2 and S3). It should be noted that since the crystal structures of the obtained powders cannot be determined in this case, such mode assignment was given under the condition that the Raman spectra of molecular powders were used to approximate the ones of isolated molecules. The Zn−BP molecule exhibits a characteristic porphyrin spectral signature, i.e., intense Raman features originating from stretching of isoindole groups and mainly involving the C atoms in a high-frequency spectral region (between 1300 and 1650 cm−1). The low-energy part of the Raman spectrum (up to 500 cm−1) consists of bands related to in-plane stretching and twisting (out-of-plane vibration) of the macrocycle. The spectral modes at 103, 136, and 481 cm−1 correspond to the inplane stretching of the Zn−N bonds, while no vibration of the isoindole units is involved. The Raman features at 250 and 586 cm−1 originate from breathing modes (symmetric Zn−N stretching) of the Zn−BP molecule. It is well established for the porphyrin and phthalocyanine molecules that most of the vibrations with energies below approximately 1000 cm−1 involve C−H out-of-plane vibrations, such as wagging and twisting, while the modes above approximately 1000 cm−1 involve C−H in-plane symmetric and asymmetric stretching motion.64,68,69 Here, according to the DFT results, the Raman modes of the Zn−BP molecule at 624 and 764 cm−1 are exclusively of a C−H wagging type. In the middle of the region of fundamental (first order) molecular vibrations of the porphyrin derivatives, one can find the modes which are related to the motion of the central metal atom. In particular, the features at 851, 894, and 942 cm−1 can be assigned to the Zn−N stretching while another at 1012 cm−1 to the Zn−N twisting. High-energy vibrations (usually above 1100 cm−1) in the porphyrin derivatives usually give the quite complicated vibrational patterns involving almost all constituent atoms simultaneously. In this respect, the Raman modes at 1115, 1126, 1163, and 1196 cm−1 are assigned to the Zn−N and C− C stretching with the C−H in-plane stretching of different symmetries. The intense Raman peaks at 1227 and 1322 cm−1, which correspond to the calculated vibrations with high Raman

located on the N atoms as well as the meso and pyrrolic carbons. Furthermore, the calculations show that the thienyl and bithiophenyl groups of Zn−TTBP and Zn−T2TBP, respectively, are rather poorly conjugated with the benzoporphyrin core, suggesting limited electronic communication between the macrocycle cores and their meso substituents. Therefore, it is important to investigate the effect of the I2 doping on the structural and spectral changes of these compounds as an alternative approach that possibly can enhance the charge transport throughout the whole macrocyclic systems. 2. Raman and IR Spectroscopic Measurements. Similarly to the porphyrins and phthalocyanines,64,65 the benzoporphyrins are large molecules having rather intricate vibrational patterns. Hence, analysis of their IR and Raman spectra is nontrivial as each mode has to be assigned to a rather complex atomic motion pattern within the molecule. Here, the Raman modes appearing due to the presence of the covalently bound thiophene-based meso substituents in Zn−TTBP and Zn−T2TBP are determined by comparing the Raman spectra of these compounds with that of the benchmark molecule ZnBP and with that obtained from the DFT calculations. Energies for these vibrational modes should be slightly different from those reported for thiophene and bithiophene molecules in the literature66,67 since, in this case, one of α-C atoms on these rings is covalently bound with the massive macrocycle. The results from the Raman studies of Zn−BP, Zn−TTBP, and Zn−T2TBP powders in a range covering their fundamental inter- and intramolecular vibrations (80−1900 cm−1) are presented in Figure 2a. Structurally, the Zn−BP molecule has 61 atoms and, therefore, should exhibit 177 normal modes (for nonlinear molecules Nnormal mode = 3Natoms − 6), while the Zn−TTBP and Zn−T2TBP molecules consists of 89 and 117 atoms, which implies the presence of 261 and 345 normal modes, respectively. Experimentally, we can identify 45 Raman-active modes for the Zn−BP molecule and observed 50 and 64 Raman modes and 41 and 44 of IR-active vibrations for the Zn−TTBP and Zn−T2TBP molecules, respectively (Figures 2b and 2c). By comparing with our DFT results, assignment of 24400


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the replacement of the H atom with the thiophene-based groups at the meso positions. Generally, the vibrational structures of Zn−TTBP and Zn− T2TBP are complicated not only because of the presence of the thienyl and bithiophenyl meso substituents, respectively, but also due to the loss of the molecular planarity as compared the planar Zn−BP. This can clearly be exemplified by the fact that some Raman modes for both Zn−TTBP and Zn−T2TBP molecules have the same spectral positions in their IR spectra, marked by dashed lines in Figure 2a and 2b. According to our mode assignment, these modes involve the respective motion of the C and N atoms, which are affected by the bending of the benzoporphyrin cores. This is also reflected in the fact that some of them have close energies for both functionalized benzoporphyrins. Due to the bending of the benzoporphyrin molecule (the loss of the initial planarity), steric interaction between the benzoporphyrin core and the meso-aryl groups took place. Consequently, the symmetry group, which describes the benzoporphyrin macrocycle, was reduced from D4h to C2(macrocycle)/C1(whole molecule). Experimentally, such bending and the resulting symmetry reduction gave rise to a change of the scattering cross section of certain vibrations and allowed normally Raman-inactive modes to become Raman active. This statement is justified by our DFT calculations, which predict the presence of both Raman- and IR-active vibrations in Zn−TTBP and Zn−T2TBP. Lastly, we would like to comment on the low-energy Raman modes of Zn−TTBP and Zn−T2TBP at 300 and 289 cm−1, respectively. As can be seen in Figure 2, such mode is not observed in the Raman spectrum of ZnBP. Therefore, it is likely that the origin of these modes is related to the meso substitution on the macrocycle. According to our DFT results, the normal modes around 300 cm−1 in Zn−TTBP and 289 cm−1 in Zn−T2TBP involve both thiophene ring twisting and Zn−N out-of-plane wagging. The shift by 11 cm−1 (300 versus 289 cm−1) toward lower energy found in the Raman spectrum of Zn−T2TBP, compared with that of Zn−TTBP, is attributed to the extension of the thiophene-based meso substitutents. Due to the larger number of the thiophene rings in Zn− T2TBP, the relative intensity of its Raman mode at 289 cm−1 is higher than that of Zn−TTBP at 300 cm−1. The only fact left open for further investigations is their strong Raman intensity, which is not anticipated, neither from the previous Raman studies of the similar molecules nor from DFT calculations for a single molecule. One of the possible interpretations is that the intermolecular interaction between the neighboring benzoporphyrin molecules enhances the intramolecular vibration related to the Zn−N wagging and/or the thiophene ring twisting. Upon I2 treatment, in situ Raman spectroscopic measurements of the Zn−TTBP and Zn−T2TBP thin films showed drastic decrease/quenching of the intensities together with red shifts and broadening of the Raman modes (middle spectra of Figures 3a and 3b, respectively). Such spectral changes are characteristic and well known for doped states of several conjugated polymers, such as polythiophene,70−72 polyaniline,73 polypyrrole,74 and related compounds, and are mainly attributed to delocalization of the molecular orbitals in polymer chains. Such charge delocalization usually takes place at the conjugated parts of the molecules, which automatically implies that the C−C and C−N bonds are affected the most. This is evident for the Raman modes in the range from 1300 to 1600 cm−1, which originate from benzene-related vibrations. Moreover, these spectral changes can suggest the polymer chain

activity, mainly include the C−N bond vibrations. The isoindole breathing mode is found at 1336 cm−1. An important vibrational mode of the Zn−BP molecule is located at 1380 cm−1 and related to meso-C−H rocking. Above 1400 cm−1, all spectral modes are dominated by the contribution of C−C or CC bonds. Here, the Raman modes at 1449, 1471, and 1573 cm−1 are benzene related. The intense modes at 1520 and 1622 cm−1 correspond to the macrocycle stretching. It should be noted that the Raman modes related to C−H stretching vibrations were also found above 2900 cm−1, but due to the complexity of their analysis, they will not be considered here in detail. In order to verify the states of the molecules upon I2 treatment, the certain spectral bands of Zn−TTBP and Zn− T2TBP related to the thienyl and bithiophenyl groups have to be addressed. These Raman features can act as markers of polymerization. By comparing the vibrational signatures of Zn− TTBP and Zn−T2TBP with those of Zn−BP, the strong Raman modes describing vibrations of the thienyl and bithiophenyl meso substituents on Zn−TTBP and Zn− T2TBP, respectively, were found at 462, 756, 1037, 1129, 1361, and 1436 cm−1 (upper spectrum, Figure 3a, blue dotted

Figure 3. (a) Raman spectra of the Zn−TTBP thin film on the ITOcoated glass substrate before (initial), after I2 treatment (doped), and after I2 removal (dedoped). Vertical lines indicate the positions of the Raman modes of thiophene (blue dotted line), bithiophene (red solid line), and I2 (green dashed line). (b) Raman spectra of the Zn− T2TBP thin film on the ITO-coated glass substrate before (initial), after I2 treatment (doped), and after I2 removal (dedoped), with the vertical lines highlighting the positions of the Raman modes of bithiophene (red solid line), quaterthiophene (violet dash−dotted line), and I2 (green dashed line).

lines) and at 378, 758, 1068, 1229, 1458, and 1550 cm−1 (upper spectrum, Figure 3b, red solid lines), respectively. The positions of most of these modes are in the range of the spectral modes of the pristine thiophene and bithiophene molecules.66−69 The difference in their energy underlines the formation of a covalent linkage between the thiophene-based units and the benzoporphyrin macrocycles in Zn−TTBP and Zn−T2TBP. This is well described by our DFT results (see Tables S2 and S3, respectively). Moreover, we also observed the spectral shift of the Raman band at 1380 cm−1, related to the meso-C−H rocking, toward lower energy at 1360 cm−1 as a consequence of 24401


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at 634, 866, 1044, 1076, and 1378 cm−1 in the case of Zn− TTBP and at 750, 1225, 1468, 1532, and 1591 cm−1 for the Zn−T2TBP molecule. The appearance of these spectral bands is attributed to the formation of new bonds during the doping− dedoping processes and is not related to I2. As for the latter, the characteristic features of the Ix−/I2 crystals were clearly absent in the spectra of the dedoped molecules. Moreover, by comparing the Raman modes observed from the pristine Zn−T2TBP film with those from the doped−dedoped Zn− TTBP one, the feature at 1044 cm−1 was detected in both cases. Therefore, it is likely that this mode corresponds to the newly formed C−C bonds of the bithiophenylene linkers in the doped−dedoped Zn−TTBP film, and therefore, polymerization of Zn−TTBP occurred during the doping−dedoping process. In a similar manner, the new features in the Raman spectrum of the doped−dedoped Zn−T2TBP film are a result of the formation of quaterthiophenylene bridges in the Zn− T2TBP film and were found in the same range as those observed for the isolated quaterthiophene molecules.77 The polymerization of Zn−TTBP and Zn−T2TBP was also studied using the FTIR technique, which is complementary to the Raman spectroscopy. Opposite to Raman scattering, IR absorption accounts for the effect of the light interaction with the dipole moments in molecules, which makes it very powerful and widely used to characterize the polymers in the pristine and/or doped states. The high sensitivity of IR spectroscopy to asymmetric molecular vibrations makes this technique suitable for characterization of the doping-induced structural changes in the conjugated organic materials since the radical formation is always connected with the decrease in the molecular symmetry. This can be seen especially well by considering the differential spectra obtained by using the ATR mode,78 which simplifies the setup for the in situ measurements. In the differential spectra, we can usually see a set of positive and negative peaks related to the emerging and vanishing spectral features due to the change in the vibrational structure upon doping. The positive peaks are also known as IR activated vibration (IRAV) bands indicating the possible formation of the radicals. The negative peaks correspond to vibrations which are suppressed (or significantly shifted) by the doping. Additionally, the FTIR spectral range can be used to detect the optical transitions related to generated polarons or bipolarons. In our studies, the Zn−TTBP and Zn−T2TBP films drop casted on the ZnSe crystals were studied in situ under I2 atmosphere in the ATR-FTIR setup. The resulting differential IR spectra were analyzed in the spectral region between 700 and 7000 cm−1 (Figure 4). The spectral range between 700 and 1700 cm−1 exhibits the vibrational structures of the Zn−TTBP and Zn−T2TBP molecules in the films affected by the presence of I2 (insets of Figures 4a and 4b). For both Zn− TTBP and Zn−T2TBP films, a series of very narrow positive and negative peaks appeared in the range between 700 and 1700 cm−1. The negative peaks of Zn−TTBP are located at 772, 812, 843, 859, 899, 1035, 1050, 1121, 1137, 1155, 1196, 1232, 1332, 1396, 1446, and 1493 cm−1, while those of Zn− T2TBP were found at 719, 759, 768, 795, 827, 879, 886, 904, 917, 944, 1034, 1136, and 1337 cm−1. According to the normalmode assignment, these features are mainly connected to the vibrations of the Zn−N bonds and partly to those of aromatic C−S−C and C−C bonds. Such trend suggests that the presence of I2 did not only affect the vibrations of the thiophene-based meso groups and/or conjugation system of the macrocycles but also the vibrations involving their metal

formation, while in contrast, the isolated molecules generally exhibit different manifestation of the changes in the Raman spectrum upon doping or radical formation, such as the appearance of new narrow Raman features which are related to localized structural modifications.75 Since the peak red shifting and broadening were not observed for the benchmark Zn−BP molecule, it is thus another indication that such spectral changes are a result of possible polymerization of Zn−TTBP and Zn−T2TBP in the film. Besides the quenching of the Raman modes, we observed the appearance of new Raman modes at 104, 165, and 325 cm−1, indicating formation of I2 crystals in the thin films. The assignment of these modes was performed by comparing the resulting Raman spectrum with that of the pristine I2 crystal and with previously reported results.76 The presence of a second-order phonon mode at around 325 cm−1 indicates the rather high crystallinity of I2 found in the film. Due to the drastic decrease of the intensity of the Raman bands during the doping process, the analysis of the vibrational structures of the doped Zn−TTBP and Zn−T2TBP is complicated. Nevertheless, for the most intense Raman bands at 1498 and 1526 cm−1, which correspond to the CC bond stretching of the Zn−TTBP molecule, in addition to a decrease in intensity, a clear shift to 1495 and 1521 cm−1, respectively, was observed in the spectra of the doped state. Such a shift toward lower vibrational frequencies is an indication for the softening of the corresponding bonds. The rest of the spectral bands of Zn−TTBP could not be monitored since they completely vanish in the presence of I2 (Figure 3a). The same behavior was observed in the case of the Zn−T2TBP molecule; only intense modes of the same origin are still visible in the spectrum of the doped species, namely, the bands of the pristine Zn−T2TBP molecule at 1495 and 1519 cm−1 merged into one broad band, centered at 1515 cm−1. It is noteworthy that the bands at 1330 and 1357 cm−1, however, did not follow the general trend of the softening and shifted toward higher energy to 1337 and 1376 cm−1, respectively. Nevertheless, line widths were found to be obviously increased, indicating a delocalization of the electronic change of the involved bonds. These bands originate from the C−C stretching in thiophene (band at 1330 cm−1) and from the benzene stretching in the macrocycle (band at 1357 cm−1). In this case, a shift toward higher frequencies (opposite to the case of CC bonds) can only be caused by benzene/macrocycle deformations. As mentioned above, I2 treatment is expected to induce polymerization of the Zn−TTBP and Zn−T2TBP molecules. Therefore, the Raman spectra of the polymerized molecules should contain spectral bands corresponding to the new species formed upon polymerization. However, as it can be seen in Figure 3, the Raman spectra of the doped molecules exhibit only intense bands related to the benzoporphyrin macrocycle. The weaker (bi)thiophene-related features are not visible. In order to clearly observe all possible Raman modes, we performed an ex situ dedoping of the I2-treated Zn−TTBP and Zn−T2TBP films. The dedoping process was performed by annealing the I2-treated benzoporphyrin films at 200 °C for 15 min or until the Raman features of the I2 crystals fully disappeared. Images of the ZnBP, Zn−TTBP, and Zn−T2TBP films before and during doping and after dedoping are shown in Figure S1. The results reveal that the Raman patterns of the pristine molecules recovered, i.e., the characteristic modes shifted back to the initial positions and became narrow and intense again (Figure 3), while the new Raman modes appeared 24402


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The effect of the charge localization can be additionally investigated from IR analysis in the spectral region above 1700 cm−1. Besides a few narrow peaks related to the bond vibrations, a broad absorption above 2000 cm−1 was observed in the case of Zn−T2TBP. Such a spectral feature in the doped polymer usually indicates the polaron formation.53 It is thus considered as a spectroscopic signature of the presence of the free charge carriers in the doped polymers. Moreover, our results reveal that the formation of the polarons was observed only for the molecule functionalized with the bithiophenyl meso groups, i.e., Zn−T2TBP. This result supports the abovementioned conclusion on the polymerization of this compound, giving rise to the quaterthiophenylene linkers, and is consistent with previous explanation on polaronic defect observed for oligothiophenes but not for bithiophene.79 From the chemistry point of view, the presence and absence of the polaron features in the polymer system can be understood from some resonance structures of the polymeric radical cations possibly generated from the oxidation process as shown in Figure 5. There are several possibilities for I2 to interact with the benzoporphyrin macrocycles and/or the oligothiophenylene linkers in the polymeric network. Therefore, the results observed from our spectroscopic measurements can be a sum of the oxidation processes at either or both sites. On the basis of the existence of the above-mentioned negative peaks in the differential ATRFTIR spectra of both Zn−TTBP and Zn−T2TBP films, the charge or radical seems to be localized on the metal center of the macrocycle. At the same time, unlike the case of the bithiophenylene linker, the delocalization of the free charge carrier generated upon I2 treatment was observed on the quaterthiophenylene unit, leading to formation of the localized polaron between the adjacent benzoporphyrin macrocycles. These approaches can simultaneously explain our Raman and ATR-FTIR results. Moreover, the Raman results are also consistent with those observed previously for K-doped Mn− phthalocyanine molecules, also showing the localized structural modifications by introducing additional charge into the molecules.78 As mentioned above, the polaron formation affects the optical absorption behavior. Usually two new optical transitions appear below the optical band gap, where one is located in the

Figure 4. In situ ATR-FTIR differential spectra during exposure of the (a) Zn−TTBP and (b) Zn−T2TBP films to the I2 vapor. (Inset) Magnification of the region between 700 and 1700 cm−1.

centers. The spatial distribution of the structural changes was also justified from the overall line shape of the IR features. Both positive and negative bands are rather narrow, indicating the formation of strongly localized charge carriers. At first glance, these IR results may look contrary to those from Raman spectroscopy, presented in Figure 3, where broadening of the spectral features was observed and attributed to delocalization of molecular orbitals due to polymer formation. However, this can be easily explained in the case of doping of the conjugated polymers, where Raman spectroscopy is more sensitive to the change in the electronic configuration, while IR spectroscopy monitors modification of the vibrational structures induced by the charge carriers. Such complementarity of the spectral changes in IR and Raman spectra is observed for both Zn− TTBP and Zn−T2TBP molecules.

Figure 5. Some possible resonance structures proposed for the polymerized Zn−T2TBP with doping occurring (a) within the quaterthiophenylene spacer and (b) between the meso-thiophene ring and the benzoporphyrin macrocycle. 24403


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Zn−TTBP film, such a separated absorption peak was not observed but a decrease in the optical band gap instead, which is related to the broadening of the optical transition distribution. This occurs most likely due to the layer thickness increase caused by I2 crystal growth inside the film, suggested by the bands of I3− observed in the Raman spectra (Figure 3). This explanation was confirmed by the spectroscopic ellipsometry. The ellipsometric angles ψ and Δ of Zn−TTBP and Zn−T2TBP layers were measured and fitted using a generic oscillator model. As a result of the optical modeling, the complex dielectric functions of the pristine and the doped layers of the molecules were obtained and are presented in the Supporting Information (see Figure S2). In accordance to the UV−vis spectra in Figure 6, the appearance of several new oscillators related to the polaron formation was observed in the case of Zn−T2TBP upon doping. As for Zn−TTBP, the generic oscillator model used to describe the pristine Zn− TTBP was successfully extended for modeling the I2-treated layer with a modified oscillator broadening, while the peak positions were preserved.

middle IR and the other is found in the NIR−vis spectral ranges. On the other hand, formation of the bipolaron results in the appearance of only one new optical transition.80,81 Thus, to understand the nature of the newly formed charged species, there is a necessity to measure the optical absorption in a wide spectral range. Therefore, in situ UV−vis spectroscopy was employed to monitor the optical transitions of Zn−TTBP and Zn−T2TBP films upon I2 treatment. The resulting serial UV− vis spectra are shown in Figure 6.

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CONCLUSIONS

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ASSOCIATED CONTENT

The doping of the target benzoporphyrins bearing polymerizable thienyl and bithiophenyl meso-groups via I2 treatment was studied in comparison to their benchmark mesounsubstituted derivative. While being doped, the Raman and FTIR spectroscopic analysis showed that both materials exhibited drastic changes in the electronic structure, such as the delocalization and renormalization of the molecular orbitals. Moreover, I2 treatment also led to polymerization at the thiophene-based sites of both compounds. In the case of the meso-bithiophenyl-substituted derivative, where the quaterthiophenylene spacer was created in the polymerization process, the polaron formation was detected by the in situ ATR-FTIR measurements and supported by the UV−vis spectroscopy and ellipsometry. The polaron formation was found to strongly depend on the chemical structures of the newly formed polymers. This work, therefore, discovered the phenomenal effects of the I2 doping on the benzoporphyrins bearing the thiophene-based meso functionalization that has paved the way for several possible applications in the field of photophysics and photochemistry.

Figure 6. Absorption spectra of the (a) Zn−TTBP and (b) Zn− T2TBP molecular films obtained during I2 treatment (in situ). Arrows indicate the major spectral changes observed during the experiment.

In the UV−vis spectrum of the pristine Zn−TTBP film a set of absorption bands at 314, 388, 478, 540, 621, and 672 nm was observed. The narrow shape of these bands is characteristic of the highly localized orbitals of the small molecules. In the presence of the I2 crystal or at the beginning of the doping process, a strong decrease in intensities and broadening at 478, 540, 621, and 672 nm were observed, while the peaks at 314, 388, and 540 nm became more prominent. The change in the peak intensities corresponds to the spatial redistribution of the molecular orbitals, and the peak broadening is related to their stronger delocalization. In a similar manner, the spectra of the Zn−T2TBP film exhibit the decrease in intensity and peak broadening of the peaks at 486, 629, and 674 nm with more pronounced and broader peaks around 394 and 564 nm appearing upon the doping process. It is worth mentioning that during the oxidation process of both materials the absorption peaks undergo slight red shifts, most likely due to the renormalized energy of the molecular orbitals. These changes of spectral profiles upon doping are fully consistent with those described in previous studies82 and our Raman results, indicating the high sensitivity of both techniques to the changes in the electronic structures of the newly formed radicals due to the delocalization of the molecular orbitals compared to the pristine materials. When considering the spectral region below the optical band gap, where the absorption signal related to the formation of the polarons is generally observed, the Zn−T2TBP film exhibited a broad peak at 940 nm upon I2 treatment. In the spectrum of the

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06771. Labeling scheme of the Zn−T2TBP molecule, calculated geometric parameters, normal mode assignments, plots of absorption coefficients from the ellipsometry, and calculated IR and Raman spectra for the benzoporphyrin compounds of interest (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dogukan Apaydin: 0000-0002-1075-8857 Georgeta Salvan: 0000-0002-2565-9675 Patchanita Thamyongkit: 0000-0003-0261-3014 24404


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The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Ratchadapiseksomphot Endowment Fund under the Outstanding Research Performance Program, Chulalongkorn University (Sci-Super III-003) and staff mobility was carried out with funding from Grant for Join Funding, Ratchadapiseksomphot Endowment Fund. The authors would also like to acknowledge the Deutsche Forschungsgemeinschaft (DFG) research grant Towards Molecular Spintronics (FOR 1154), the European Commission under the H2020 grant TWINFUSYON (K.H.), and the Wittgenstein Prize of Austria Science Foundation (FWF) for financial support.

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